This invention relates generally to direct current (DC) voltage boosting circuits, and in particular, to a DC voltage boosting circuit that includes an inductive charge pump circuit for producing one or more regulated DC voltages useful for flash memory and other applications.
Many applications require circuits that can boost up an input power supply DC voltage to a higher DC voltage used for specialized operations. The reason for the voltage boost up is that often only standardized power supply voltages are available for supplying power to electronic circuits. However, there may situations where a circuit needs a higher voltage than one available from the associated power supply. One example of such a circuit is an electrical erasable programmable read only memory (EEPROM), typically termed in the art as xe2x80x9cflash memory.xe2x80x9d
A flash memory generally comprises an array of memory cells each typically storing a bit of digital information. Often, a memory cell is merely a field effect transistor (FET) that includes a floating gate which holds a charge that corresponds to a bit of digital information (termed herein as a xe2x80x9cbit chargexe2x80x9d). More specifically, a memory cell FET comprises a drain, gate, and a source, wherein the gate includes a control gate for enabling reading, writing, and erasing operations on the cell, and a floating gate for storing the bit charge of digital information. In addition to these gates, some memory cells include an erase gate used for removing the bit charge from the floating gate, thereby erasing the memory cell.
The writing (i.e. programming) of a bit charge of digital information typically involves electrons that tunnel or inject from the FET channel through the thin gate oxide to the floating gate. Generally, the tunneling effect or hot electron injection requires relatively high energy to move the electrons across the gate oxide layer. Similarly, the removing of electrons from the floating gate during an erase procedure requires relatively high energy to move the electrons across the gate oxide or an oxide situated between the erase and floating gates. The source for the high energy for both the writing and erasing operations is typically a relatively high voltage source, which is higher than the power supply voltages used for memory control operations. For example, the voltage required for writing and reading operations may be on the order of six (6) volts, whereas the voltage for the memory control normal operations may be on the order of 1.5 volts.
In a typical flash memory circuit, the majority of the circuit operations require a voltage on the order of 1.5 volts, for example. Thus, the design of the power supply for the flash memory circuit includes a 1.5 volt power supply. However, to generate the higher voltage used for writing and erasing operations, a DC voltage boost circuit is used which takes the normal power supply voltage of 1.5 volts and boosts it up to about six (6) volts to perform these higher voltage operations. In general though, DC voltage boost circuit can convert any input voltage to any desired output voltage.
An example of such a DC voltage boost circuit is a capacitor charge pump circuit which includes a plurality of cascaded stages each comprising a switch and a capacitor. A previous stage in the cascade supplies charges to a next stage in boosting an input voltage to a higher output voltage. However, this type of boost circuit is typically inefficient due to the losses incurred in the transfer of charges between stages, and the losses incurred across each capacitor. The efficiency for a DC voltage boost circuit that includes a capacitor charge pump circuit is on the order of about five (5) to eleven (11) percent.
Thus, there is a need for a DC voltage boost circuit that has improved efficiency in the conversion of a relatively low input voltage to a relatively high input voltage. Such a need is provided for in the new DC voltage boost circuits of the invention, as described below. The DC voltage boost circuits of the invention can be used for flash memory, static and dynamic random access memory (RAM) application, or any other application which may or many not be related to memory applications. In general, there is a need for a DC voltage boost circuit that generates an output voltage from an input voltage.